Apparatus and a process for drying a paper web

ABSTRACT

An apparatus for drying a fibre web of paper or paperboard is provided. The apparatus includes an adjustable heated press nip including at least one pressure device. The pressure device includes a roll, supporting one side of the web and at least two pressure shoes opposite the device and acting on the other side of the web. Each of the pressure shoes include means for providing an individual pressure respectively on the web. At least one intermediate conduit is arranged in-between the pressure shoes. The intermediate conduit includes means forming an additional individual pressure zone on the web.

The present invention relates to an apparatus and a process for drying afibre web, particularly an apparatus and a process having an adjustableheated press nip containing at least one pressure device, such as aroll, and at least two pressure shoes opposite said device, where eachof the pressure shoes contains means for providing an individualpressure respectively on the web.

The process of drying a fibre web during application of pressure with aheated roll or similar pressure device, i.e. so called impulse drying,is a technique that has been under development for the recent twentyyears. Bringing a paper web in a press roll nip made the first attempts.Thereafter, a paper web was brought between a felt and a hot metalsurface while subjecting a substantial pressure to the felt. By theexpression “drying” a fibre web in accordance with the present inventionis also meant that the web is subjected to pressing, or dewatered inorder to increase its dry substance content.

Later on a slow press roll, which could be heated to temperatures above300° C., was constructed. A wet paper web and a felt were broughtthrough the nip. However, the apparatus was slow and consequently thepress period was long, resulting in a completely dry paper web. Anotherattempt to find an industrial application for impulse drying wasconcentrated on substantially increasing the dry content of paper webbefore introduction in the drying part of the paper machine. In thefirst experimental plant it was found that delamination of the paperoccurred. The explanation to the paper delamination is that when thewater content in the paper is heated above 100° C. during a highpressure, whereafter the pressure is decreased to atmospheric pressureafter the press nip, a spontaneous evaporation starts. Hence, in orderto provide space for the thus formed vapour, the paper may becomedelaminated.

Numerous proposals how to avoid delamination, have been suggested. Forinstance, the press roll to be heated has been provided with a porousceramic coating, for restricting the conduction of heat at a certaintemperature and thus the water temperature in the web. This has beencombined with several proposals of retaining the pressure a certainperiod after the nip, to give increasing time for vapour penetrationthrough the web. However, none of the above mentioned solutions presentsa satisfactory and useful solution to the problem of delamination.

U.S. Pat. No. 5,404,654 discloses an impulse drying apparatus for thetreatment of paper or paperboard web, including a pair of rolls having anip between them, where at least one of the rolls being heated. A steamchamber is located next to said nip.

U.S. Pat. No. 5,071,513 relates to a method for the mechanical-thermaldewatering of a fibre stock web, where at least two parallel rows ofadjustable pressure elements cooperate with a heatable counter roll.

U.S. Pat. No. 5,709,778 discloses a multiple shoe press for a papermaking machine. The shoe press has a shoe assembly including a pluralityof shoes disposed adjacent each other.

The purpose with the present invention is to minimise the drawbacks ofthe processes and devices disclosed in the prior art. The object is toeliminate the occurrence of delamination of a fibre web during animpulse drying. A further object is to achieve an apparatus whichoperates more efficient and can be easily controlled to an increasingextent with respect to what is known in the art.

The present invention provides an apparatus for drying a fibre web, e.g.a web of paper or paperboard, comprising an adjustable heated press nipincluding at least one pressure device, such as a roll, supporting oneside of the web and at least two pressure shoes opposite said device andacting on the other side of the web, each of the pressure shoesincluding means for providing an individual pressure respectively on theweb, where at least one intermediate conduit is arranged in-between saidpressure shoes, whereas the intermediate conduit includes means whichforms an additional individual pressure zone on the web.

An advantage with the apparatus of the present invention is that theentire pressure profile is better controlled and delamination of a fibreweb during a drying treatment can be completely avoided.

A further advantage with the apparatus of the present invention is thatif a delamination occurs, the web might be compressed again, totally orto a certain extent, and thus the delaminated area can be at leastpartly cured and repaired.

In addition, the present invention also relates to a process for dryinga fibre web, e.g. a web of paper or paperboard, where the web issubjected to a drying treatment in an apparatus having an adjustableheated press nip formed by arranging at least one pressure device, suchas a roll, supporting one side of the web and arranging at least twopressure shoes opposite said device and acting on the other side of theweb, each of the pressure shoes including means for providing anindividual pressure respectively on the web during the drying treatment,and arranging at least one intermediate conduit in-between said pressureshoes, and providing the intermediate conduit with means serving as anadditional individual pressure zone towards the web, and subjecting theweb with an additional pressure during the drying treatment.

The press nip according to the present invention involves means forproviding heat to the fibre web. The pressure device, roll or counterroll is heatable. Said pressure device has means for providing atemperature exceeding 100° C. in the heated press nip. When the fibreweb is subjected to a temperature reaching the evaporating point at thepressure used (i.e. at least exceeding 100° C.), the water in the fibreweb will vaporise and the steam produced must necessarily be providedspace due to its expansion from water to steam. This may occur insidethe web and the web might as a consequence become delaminated. Onereason why the operation of compression and curing of a web can becarried out, in accordance with the present invention, is due to thefact that the original structure of the paper is not completely damagedwhen delaminated into two layers of a web. A suitable upper limit forthe temperature applied in the drying treatment can be about 500° C.

The application of a compressive pressure in the press nip may suitablyexceed about 0.5 MPa. The outermost upper limit for application of thepressure depends on several conditions, but may be about 10 MPa. Themaximum practical pressure applied by the pressure shoes, can be about6-7 MPa and the pressure can be gradually decreased to atmosphericpressure. The corresponding longitudinal pressure of a roll, or apressing device, in the press nip in accordance with the presentinvention, is at least from about 100 kN/m while not exceeding about1500 kN/m.

It is also within the scope of the invention to arrange a conveyingbelt, e.g. a steel belt, in between a roll and an opposite pressuredevice. The conveying belt can serve as support for the web and the beltcan serve as the heating zone used for providing the desired temperaturein the heated press nip.

The press nip according to the present invention comprises of at leastone pressure device, such as a roll, supporting one side of the web.According to the present invention, said device can be an ordinary axialarranged substantially solid roll, preferably made of steel. By theexpression “roll” is also meant a hollow cylinder where only the surfacerepresents the shape of a roll. Inside the hollow cylinder body one ormore pressure element/elements can be arranged so as to act as pressureelements against the inner walls of the hollow cylinder and consequentlyperforming the desired pressure against the web, placed on the oppositeside of the cylinder wall.

On the other side of the web in relation to at least one pressuredevice, or the preferred roll, the web is in corporation with at leasttwo pressure shoes opposite said pressure device and acting on the otherside of the web. The number of pressure shoes used can be up to five, oreven more. The number of pressure shoes applied is only limited bypractical reasons and depends on the length of the desired total presszone of the nip and the size of each shoe. Each of the pressure shoeshas preferably an extension in the transverse direction substantiallyequal to the width of the transported paper, i.e. along the width of thepaper web. Each of the pressure shoes may include means for providing anindividual pressure on the web respectively. The means can be mechanicalmeans with a fluid acting as a pressure medium, for example oil or asimilar fluid. Preferably, oil is used as medium for said means. Thepressure shoes are acting either according to the hydrodynamic or thehydrostatic principle. Hence, in accordance with the hydrodynamicprinciple, a continuous fluid flow is provided to the pressure shoes. Ifacting with the hydrostatic principle, each pressure shoe is providedwith a fluid from separate chambers. The upper surface of the pressingshoes, in the area facing the web, may be inclined towards the web.Hence, each pressure shoe acting against the web may form a convergingpress nip.

For the purposes of maintaining a minimum load level in the spacebetween the pressure shoes and thereby avoid a risk for delamination, apressure level is applied even between the pressure shoes. Hence,according to the apparatus and the process of the present invention, anintermediate conduit is arranged, or formed, in-between at least twoadjacent pressure shoes. Preferably, each intermediate conduit has anextension in the transverse direction, i.e. along the width of the paperweb, substantially equal to the width of the pressure shoes. The wallsand/or surrounding surfaces of the intermediate conduit, which also canbe called channel, spacing or the like, may constitute of a specificconstructional shaping of its own or formed just by the surroundingsurfaces of the pressure shoes. The intermediate conduit includes means,which forms an additional individual pressure zone on the web. Saidadditional individual pressure zone includes means for providing anindividual pressure respectively on the web. Said means can bemechanical means with a fluid acting as a pressure medium, for exampleoil or a similar medium. Preferably, oil is used as medium for saidmeans. Like the aforementioned pressure shoes, said additional pressurezone is acting either according to the hydrodynamic or the hydrostaticprinciple. With this new design, the entire delamination can becontrolled to a higher degree. It is possible, for instance, to apply areversed wedge pressure profile, in order to relieve the applied loadslowly. Thus, even a slight delamination may occur in said conduits, inthe intermediate space between the pressure shoes. The pressure in theintermediate space between each pressure shoe can be relievedsignificantly, by using a low main or average pressure level, P_(m).Generation of flashed steam due to overheated water would occur in everypressure shoe, but to a smaller extent, since the residence time in eachpressure shoe is shorter. The amount of flashed steam per nip is smallerand this can possibly flow out of the sheet into the felt withoutdelaminating the web. It may still be possible if delamination occurs,to partly or even completely, compress and repair the web again, inaccordance with the apparatus and the process of the present invention.

Each intermediate conduit is suitably connected to a common fluidchamber. The intermediate conduit and the fluid chamber form together aclosed compartment, serving as an additional pressure element.Intermediate conduits can be arranged between only one pair of pressureshoes or between every adjacent pair of pressure shoes. Intermediateconduits may also be arranged on the outside of the respectively outmostplaced pressure shoe.

The pressure shoes, as well as the additional pressure element, can bemade of steel or similar material.

With respect to the pressing time for the web in the press nipsrespectively, said tine may vary from parts of a second up to severalseconds, within the used temperature and pressure ranges as mentionedherein.

The fibre web subjected to a drying treatment according to the presentinvention is preferably a paper web with substantial content oflignocellulose-containing fibres.

With reference to the accompanying drawings, embodiments of the presentinvention are described, without restricting the scope of the presentinvention thereto.

FIG. 1 shows a side view of a principle illustration of an apparatus fordrying a fibre web in accordance with the present invention.

FIG. 2 illustrates in a diagram a possible pressure profile of theapparatus in FIG. 1 during a drying operation.

In FIG. 1, a principle illustration of an apparatus 2 for drying a fibreweb 4 is shown. An adjustable heated press nip 6 is formed in a spacebetween a heated roll 8 and pressure shoes 10, opposite the roll 8. Saidroll 8 is supported and arranged towards one side of the web 4, andpress shoes 10 are acting with varying pressure towards the other sideof the web 4. The fibre web 4 is arranged for a continuous travellingover the roll 8, also called backing roll or counter roll. As evidentfrom FIG. 1, five press shoes 10 are arranged against the web 4. Each ofthe press shoes 10 includes means for providing an individual pressureP₁, P₂, P₃, P₄ and P₅, respectively, on the web 4. A flow of oil isgenerally indicated at 12. At least one intermediate conduit 16 isarranged in-between two of said pressure shoes 10, whereas theintermediate conduit 16 includes means 18, 20 which forms an additionalindividual pressure zone 24 on the web 4. As evident from the diagram inFIG. 2, a possible pressure profile of the apparatus in FIG. 1 during adrying operation is shown. The vertical axis in the diagram representsthe added pressure, P. The horizontal axis represents the position, X,in the press zone. In the diagram, five peak pressures (P₁, P₂, P₃, P₄and P₅) are shown, which corresponds to the respective pressure zone ofeach pressure shoe in FIG. 1 with the pressures P₁, P₂, P₃, P₄ and P₅.Furthermore, as evident from the diagram, a main pressure level, P_(m),is provided between each of the peak pressures (P₁, P₂, P₃, P₄ and P₅).The P_(m) is provided by the additional pressure zone 24 throughintermediate conduits in between each pressure shoe.

What is claimed is:
 1. An apparatus for drying a fibre web of paper orpaperboard, comprising an adjustable heated press nip including at leastone pressure device, comprising a roll, supporting one side of the weband at least two pressure shoes opposite said device and acting on theother side of the web, each of the pressure shoes including means forproviding an individual pressure respectively on the web, wherein atleast one intermediate conduit is arranged in-between said pressureshoes, and wherein the intermediate conduit includes means forming anadditional individual pressure zone on the web.
 2. An apparatus fordrying a fibre web according to claim 1, wherein said means forming anadditional individual pressure zone includes a fluid acting as apressure medium.
 3. An apparatus for drying a fibre web according toclaim 1, wherein each said intermediate conduit is connected to a commonfluid chamber, the intermediate conduit and the fluid chamber togetherforming a closed compartment, serving as the additional pressure zone.4. An apparatus for drying a fibre web according to claim 1, wherein thetotal pressure acting on the web is arranged to provide a pressure in arange from about 0.5 MPa up to about 10 MPa.
 5. An apparatus for dryinga fibre web according to claim 1, wherein heated press nip has means forproviding a temperature exceeding 100° C. in the heated press nip.
 6. Anapparatus for drying a fibre web according to claim 1, wherein aplurality of intermediate conduits are respectively arranged betweenevery adjacent pairs of a plurality of pressure shoes.
 7. A process fordrying a fibre web of paper or paperboard, wherein the web is subjectedto a drying treatment in an apparatus having an adjustable heated pressnip formed by arranging at least one pressure device, comprising a roll,supporting one side of the web and arranging at least two pressure shoesopposite said device and acting on the other side of the web, each ofthe pressure shoes including means for providing an individual pressurerespectively on the web during the drying treatment, comprising thesteps of arranging at least one intermediate conduit in-between saidpressure shoes, providing the intermediate conduit with means serving asan additional individual pressure zone acting towards the web, andsubjecting the web to an additional pressure during the dryingtreatment.
 8. A process for drying a fibre web according to claim 7,wherein pressure is suitably in a range of from about 1 MPa to about 10MPa.
 9. A process for drying a fibre web according to claim 7, furthercomprising the step of providing means in the heated press nip to affecta temperature exceeding 100° C.
 10. A process for drying a fibre webaccording to claim 7, wherein the additional pressure is provided by theadditional pressure zone through intermediate conduits in between eachpressure shoe.